Update documentation on installation of python-igraph

[vtraag]

A user reported in the forum that after installation of python-igraph using conda it could be checked that it was properly installed by doing

import igraph.tests
igraph.test.run_tests()

However, this has never worked like that, at least not since python-igraph 0.8.0 (as far as I could check).

This PR fixes that issue.

There are some other issues that we should also address:

• The section on installing python-igraph from PyPI also mentions running the tests using python -m unittest discover. @ntamas, if I'm correct, this does not work, right? As far as I can see, no test files are included in any of the binary wheels at least, only in the source tarball.
• All of the commands (i.e. leading with $) are being rendered in normal font. This also messes up some commands, as it renders -- as an em-dash –. I think the code blocks should be introduced with double colons instead of a single colon?
• Some links are incorrectly formatted, most likely because of line-breaks in the .rst.
• Pieces of code also seem to be incorrectly rended, and should be within double backticks, not single backticks I believe.

I can further correct these tomorrow.

[ntamas]

The section on installing python-igraph from PyPI also mentions running the tests using python -m unittest discover.

You are correct, this is outdated. It worked for the source tarball some years ago but probably not any more (and I plan to migrate to pytest in the long run anyway). This can be removed from the instructions. Also, we don't ship unit tests any more with python-igraph

You are also correct in all the other formatting-related issues.

[vtraag]

I've now only corrected the formatting for the install page, but we should still correct the formatting for the entire user guide. I've opened issue #447 for that to not loose track.

[vtraag]

AFAIC this can be merged.

[ntamas]

Thanks a lot!